In oil and gas wells, a “drive string” connects the pump, located down hole, to the drive system, located at the surface. Conventional sucker rods are elongated steel rods, 20 feet to 30 feet in length. Traditional drive string typically consisted of a sequence of conventional sucker rods with connecting mechanisms at each end of each conventional sucker rod which permit end-to-end interconnection of adjacent rods. In contrast, continuous sucker rod is a unitary rod, consisting of one elongated continuous piece of steel. Thus, continuous sucker rod does not have the numerous interconnection points found in the interconnected conventional sucker rods. Each interconnection point between two successive conventional sucker rods is a source of potential weakness and excess wear on the adjacent tubing and casing. However, increased costs can be associated with continuous sucker rod.
The length of a drive string can vary from anywhere from as little as 500 feet to as much as 10,000 feet or more, depending on the depth of the well and desired location of the pump down hole. Continuous sucker rod is typically produced and stored for sale on large transport reels. These transport reels have a maximum diameter of about 19 to 20 feet and the diameter may be as small as 9-10 feet. (The desired maximum diameter is limited by transport issues). A full reel can carry continuous sucker rod with lengths of over 6,000 feet depending on the diameter of the rod.
The properties of the steel used for any drive string sucker rod, whether continuous or conventional sucker rod depend upon the conditions of the well and the drive system and pumping system used to produce the well. Sucker rod is generally classified into grades which are suitable over a range of load conditions and/or environmental conditions, such as H2S content of the well. The design of continuous sucker rod must be such that the continuous sucker rod can be wound tightly enough to fit snugly on the transport reel and then be able to be straightened into a drive string at the well, without sacrificing the desired properties for the load and environmental conditions of the intended use. Winding the continuous sucker rod onto transport reels sometimes causes permanent deformation as the rod is wrapped onto the transport reel and then straightened in the field for use.
Palynchuk, Canadian Patent No. 942,585 discloses one of the original methods of manufacturing continuous sucker rod. In Palynchuk, continuous sucker rod was made by taking a series of input coils, joining the ends of the coils together, and subjecting the joined coils to a series of treatment steps. The coils were also hot worked from a round cross-section to oval cross-section along the entire length of the continuous sucker rod. The oval cross-section permitted the continuous sucker rod to be wrapped on the transport reel in the direction of its minor diameter which reduced the extent of permanent plastic deformation in the continuous sucker rod. Oval cross-sectional continuous sucker rod is generally used with reciprocating pump applications.
Heavy oil wells are most often produced with progressive cavity pumps (“PC Pumps”). PC Pumps are driven by a rotary drive and consequently, the drive string used in these applications also rotates. Oval cross-section sucker-rod is not suitable for rotating drive string applications due to the eccentric loads encountered during rotation and greater wear caused along the tubing. Also, the effects of plastic deformation on sucker rod performance are less of a concern with rotating drive strings because the loads are torsional and the rotating drive strings are not subjected to the cyclical high compression/tension loads experienced in the reciprocating pump applications. Therefore, expensive oval cross-section continuous sucker rod, such as that disclosed by Palynchuk, is not generally used for rotary drive applications. Round cross-sectional sucker rod and continuous sucker rod, is more suitable.
Steel used to make continuous sucker rod is received from the steel mill in raw coils. The steel is manufactured by the steel mill to meet specifications as directed by the sucker rod manufacturer. Steel manufactured to ASTM standard A576 and supplementary requirements S7, S8, S11, S12 and S18 is known to produce suitable sucker rod for most oil and gas applications. To meet these requirements, the input coils are specially alloyed using known techniques to produce a grade of steel with suitable hardenability, strength, toughness, corrosion resistance, fatigue resistance, micro-cleanliness, and weldability.
However, the hardness and corresponding tensile strength of the steel coils received from the mill in raw form is inconsistent, highly variable along individual coils and from coil to coil, and relatively low. Since tensile strength is one of the most critical requirements for all sucker rod, it is necessary for the entire length of the steel coils to be subject to treatment during the manufacture of the continuous sucker rod to ensure that the critical tensile strength requirements are met and are uniform along the length of the continuous sucker rod. Input coils received from the steel mills in prior art practices are generally of very low hardness due to the chemistry and manufacturing processes used in the steel mill.
Usually, a number of the raw coils must be fused together end-to-end to form one continuous sucker rod of the desired length. The ends are usually fused together by welding which creates heat-affected zones adjacent to the welded area which must be treated to relieve stresses and yielding caused by the welding process. Without such treatment, the heat-affected zones would be a source of potential weakness which could cause failure of the continuous sucker rod in use.
Prior art methods treat the entire length of the rod with a series of austenizing, quenching, and tempering treatment steps which produce a final continuous rod which is of consistent hardness and strength and which also alleviate the problems induced by the welding in the heat-affected zone. The rod must be straightened and many of these steps are to be applied along the entire length of the rod. Usually, two or three successive production lines are required to subject the continuous sucker rod to all of the necessary steps, with the rod being uncoiled, straightened, treated as it passes through each line, coiled, transferred to the beginning of the next line, uncoiled and straightened to pass through the next line, and so on.
These prior art methods of manufacturing continuous sucker rod therefore require extensive heavy, permanent equipment and a large fixed facility to practice the method within. Steps such as ambient cooling necessitate a long open space within the manufacturing facility to permit the length of rod to be exposed for the requisite period of time and some present facilities in which the prior art methods are practiced can be as long as 300 feet or more. As a result, these prior art methods involve significant capital investment.
Recent methods have sought to reduce this capital investment by using 40 foot rods transported directly to the well site and fusing them together with a “portable” plant at the well site itself (see Widney et al, CA P 2,317,291). Such methods are disadvantageous in that they are highly labour intensive at remote locations.
What is needed then is a method of manufacturing continuous sucker rod which reduces the number of treatment steps required to be performed without sacrificing essential properties required to make the rod suitable for load and environmental conditions as specified. It would also be preferable to have a method which permits reduced capital investment into equipment and facilities, thereby reducing costs.